It is known to brake vehicles by causing the asynchronous motor to function as a generator and by dissipating the electrical power generated in resistors or a rheostat. The majority of manufacturers prefer this type of electrical or rheostatic braking to mechanical braking since it allows the maintenance costs to be minimised for wearing parts on these vehicles.
During a rheostatic braking operation, if the DC bus is connected to the catenary, a return of energy towards the catenary is seen. An energy return of this type is not acceptable for some users.
On the other hand, if the DC bus is disconnected from the catenary, the direct voltage on this bus progressively decreases and becomes insufficient to magnetise the motor so that a rheostatic braking operation is no longer possible.
In order to overcome this problem in a free wheel mode, during which the vehicle coasts (free wheeling) and the DC bus is disconnected from the catenary, known methods maintain a direct magnetisation voltage on this DC bus by carrying out a pre-braking phase.
This pre-braking phase comprises:    a) a step for magnetising the motor from the direct voltage present on the bus, then    b) a step for operating the motor as a generator of alternating voltage and operating the converter as a rectifier for the alternating voltage in order to recharge the bus with direct voltage.
In known methods, step b) lasts for as long as the vehicle is coasting and the DC bus is disconnected from the catenary. The motor therefore functions constantly as a generator in order to maintain an adequate level of magnetisation voltage on the DC bus.
This has the consequence of permanently imposing a deceleration of the electric vehicle, even if it is coasting.
The object of the invention is to overcome this disadvantage by providing a method which is capable of minimising the deceleration imposed on the electric vehicle in order to maintain a magnetisation voltage on the DC bus.